The invention pertains to the field of semiconductor fabrication. More particularly, the invention pertains to the detection and monitoring of striations in multi-layer film stacks.
Hundreds of processing steps, known to those skilled in the art, are typically required to fabricate integrated circuits on semiconductor substrates. The integrated circuits are created from multiple layers of various materials, semiconductors, oxides and metals, which are deposited onto thin disks of purified crystalline semiconductor substrate material, typically silicon, germanium, gallium arsenide or other materials known to those skilled in the art. These multiple layer film stacks are called wafers. After, processing, wafers are divided into separate chips.
The oxide or dielectric layers are used as insulation between conducting layers, as well as a source of dopants for diffusion, getters for impurities and passivation to protect devices against impurities, moisture and physical damage. Dielectric materials typically are used as multi-layered film stacks containing films varying in structure or composition. Commonly used dielectric materials include phosphorous-doped silicon dioxide or phosphorous silicate glass (xe2x80x9cPSGxe2x80x9d) and boron phosphorus silicate glass (xe2x80x9cBPSGxe2x80x9d).
One type of defect that is especially prevalent in phosphorous-doped oxide layers in multi-layer film stacks is a striation. Striations are localized areas with high concentrations of phosphorus which can not effectively be removed during further processing and are fatal to the product wafers. Striations are formed during deposition of the oxide layer as a result of processing problems, such as when one or more liquid phosphorus injector(s) on the production line become clogged or when condensation and vaporization occur due to cold spots in the line during chemical vapor deposition.
During processing of product wafers, oxides that are used as insulators between conducting layers are deposited and then etched to open windows for electrical connections, and oxides that are used as passivation for devices are etched to open areas for bonding. A fluoride containing solution or plasma, such as a hydrofluoric acid solution or a CHF3 plasma, is used to etch the oxide. However, the rate of etching is phosphorous-dependent such that areas with higher phosphorous concentrations etch more rapidly than areas with lower phosphorous concentrations. An example of the deleterious effects of striations follows. A window is cut into the product wafer so that all of the layers are exposed, after which the fluoride containing etch is performed. The striations are etched at a higher rate than other areas of the product wafer, leaving a void or crack in the oxide layer. When metal is later added to the product wafer during further processing, the metal fills the hole created by the striation, negating the insulating properties of the oxide layer and breaking the current running through the product wafer, thereby effectively destroying the product wafer. Accordingly, product wafers containing striations can be detected during electrical testing of finished wafers and such product wafers are scrapped.
In large-scale production of integrated circuit devices, control wafers are used to detect defects which occur during the manufacturing process. Control wafers are generally used to check for film composition, film structure and film contaminants. Control wafers are processed separately from product wafers, typically prior to running product wafers and at regular intervals during the fabrication of product wafers. The control wafers are destructively evaluated to detect defects in the film layer stacks prior to additional production of product wafers. Using specialized control wafers as test wafers facilitates the early detection of processing problems and prevents the need to scrap entire production lines of product wafers.
Striations can only be detected in the layered oxide film stack by using destructive analytical techniques. Even if processing is interrupted to analyze the freshly deposited oxide film for striations, there is no technique available that will allow the detection of localized concentrations of phosphorous throughout the film thickness without sectioning the wafer or xe2x80x9cburningxe2x80x9d a hole into the wafer. For example, x-ray fluorescence spectrometry (XRF), a metrology tool which is utilized on the production line, will only detect the phosphorus levels vertically down through the entire wafer by averaging the amount of phosphorus it detects throughout the total thickness of the wafer. XRF is, therefore, incapable of reliably detecting localized areas of phosphorus. Since the localized concentrations of phosphorous may only be detected using destructive techniques, control wafers are used for the detection of striations rather than destroying product wafers.
Scanning electron microscopy (SEM) and secondary ion mass spectroscopy (SIMS) are alternate methods for the detection of striations in control wafers which each require the wafer to be taken off the production line for testing. For SEM analysis, the control wafer is cross-sectioned and then etched with a fluoride containing etch. Areas with higher concentrations of phosphorous etch faster than other areas, creating voids or cracks so that the striations appear as dark areas in the SEM. SIMS functions by sending an oxygen beam through the thickness of the control wafer to dislodge atoms from the material by collision with the oxygen beam, and a detector identifies what material is at each depth of the control wafer. SIMS produces a profile of the phosphorous concentration throughout the thickness of the control wafer.
Typically, control wafers are made using the same equipment and processes that are used for making the product wafers in order to simulate the product wafers. The current method of detecting striations includes a standard production recipe for a control wafer. The standard configuration for a control wafer (5) is depicted in FIG. 1. There are four layers: a substrate layer (2), usually made of silicon, germanium, gallium arsenide or other materials known to those skilled in the art, and three additional layers that mimic the product: an undoped silicate glass (USG) layer (4), a PSG layer (6), and a BPSG layer (8). Layers (4), (6), and (8) of the control wafer (5) are generally found as intermediate layers in product wafers.
Preferably, the total thickness of the control wafer (5) is approximately the same as a finished product wafer, in order to make the control wafer (5) more representative of the product wafer. In the control wafer (5) known in the prior art, the thickness of the three silicate glass layers follows a standard recipe. The substrate layer (2), which may vary in thickness, is not discussed here. Typically, the USG layer (4) in FIG. 1 has a thickness of about 1000 xc3x85, the PSG layer (6) has a thickness of about 1500 xc3x85, and the uppermost layer, BPSG (8) varies depending on the production line and the thickness of the layers on the particular product wafers, but typically has a thickness of about 6000 xc3x85. The thickness of the PSG layer (6) on the control wafer (5) was chosen to mimic the thickness of the PSG layer on the product wafer which typically has a thickness of 1500 xc3x85 for maximum ability to getter or trap sodium impurities.
In the control wafer (5), the substrate layer (2), the USG layer (4) and the BPSG layer (8) are non-essential, that is, they have no function in the detection of striations in the control wafers. The PSG layer (6) is a phosphorous-doped oxide layer in the control wafer (5). The striations are observed in the BPSG layer. When there is a problem with the hardware on the production line during chemical vapor deposition of the PSG layer (6), striations may be produced in the PSG layer (6) and the BPSG layer (8) of both the control wafer (5) and product wafers.
The standard control wafer (5) configuration, however, does not consistently detect striations that occur within the product wafers during processing. By inspecting etched, cross-sectioned wafers using scanning electron microscopy (SEM) for the current invention, striations have been observed to occur at varying thickness levels of the PSG layer, however, the average frequency of a striation has been observed to occur at approximately 2500 xc3x85. This number is only an average, and therefore striations may occur over a large range. In fact, the range can be from 1 xc3x85 to infinite, but the striations usually range from about 2000 xc3x85-about 3000 xc3x85. Since the average frequency of a striation is about every 2500 xc3x85, however, it is unlikely that each individual control wafer (5) will definitely contain a striation when there is a processing problem, thus making the control wafer (5) ineffective at positively detecting when striations may occur in the product wafers on the production line.
If, for example, a certain striation has the potential to occur at about 2500 xc3x85, the control wafer (5) with only a 1500 xc3x85 thick PSG layer (6) would not contain the striation. Although the liquid phosphorus injectors are clogged or there is another hardware processing problem resulting in uneven concentrations of phosphorous in the PSG layer (6), the striation xe2x80x9cskipsxe2x80x9d the PSG layer (6) in the control wafer (5) because the PSG layer (6) is too thin to consistently contain the striation within the PSG layer (6). Therefore, when the control wafer (5) is tested for striations prior to or during mass production of the product wafers, a striation is not detected. Consequently, potential processing problems on the production line which can cause striations in the product wafers are not accurately represented by the control wafer (5).
Since the PSG layer (6) in the control wafer (5) is too thin to consistently contain striations, many of the control wafers are defect-free, while the product wafers contain striations. Thousands of product wafers are processed, thereby resulting in striations in at least some of the product wafers. However, the defect is overlooked and the product wafers continue through additional processing. The product wafers are tested just prior to shipment. At this time, final testing steps are performed to see if current passes through the product wafer. As discussed above, product wafers containing a striation do not effectively pass current. Typically, if a striation is found within a lot of product wafers, the entire lot must be scrapped due to future reliability concerns, wasting both time and money.
Product wafers continue to be scrapped due to the inability to regularly detect and monitor striations in multi-layer film stacks using conventional control wafers. There is a need for a better, earlier and more reliable method of detection in order to make the production of product wafers more cost effective.
Briefly stated, a new control wafer configuration and method allow for more reliable and earlier detection of striations, and the early correction of the processing problems that cause the striations. By modifying the standard control wafer recipe currently being used, processing problems are more likely to be detected prior to processing additional product wafers and prior to further processing of any defective product wafers. By increasing the thickness of the PSG layer in a film stack from about 1500 xc3x85 in the standard control wafer recipe to a thickness greater than about 2500 xc3x85, preferably a range between about 3000 xc3x85 to about 4000 xc3x85, processing problems resulting in high-localized phosphorus striations are consistently detected within the PSG layer during destructive testing of the control wafer. As a result, the PSG layer in the control wafer more accurately predicts potential defects in the product wafers. If there is a problem on the production line, the striations are detected in the control wafer before mass production of product wafers continues. Accordingly, the processing facility can be shut down to repair the processing problem before additional product wafers are produced and defective wafers do not undergo further processing, thus reducing scrap and saving additional processing time and money.
In an embodiment of the invention, a control wafer to test for striations includes a substrate layer, an USG layer deposited directly on top of the substrate layer, a PSG layer having a thickness greater than about 2500 xc3x85, preferably from about 3000 xc3x85 to about 4000 xc3x85 deposited directly on top of the USG layer, and a BPSG layer deposited directly on top of the PSG layer wherein a total thickness of the control wafer is equivalent to the total thickness of a product wafer.
In another embodiment of the invention, a method of detecting a striation in a control wafer for a multi-layer film stack includes (a) producing a control wafer wherein the control wafer comprises a substrate layer, an USG layer deposited directly onto the substrate layer, a PSG layer having a thickness greater than about 2500 xc3x85, preferably from about 3000 xc3x85 to about 4000 xc3x85 deposited directly on top of the USG layer, and a BPSG layer deposited directly on top of the PSG layer wherein a total thickness of the control wafer is equivalent to a total thickness of a product wafer, and (b) analyzing the control wafer to detect any striations.